Python won't recognize NULL (or None)

09-26-2011 01:11 PM
Occasional Contributor
Trying to calculate a field based on another field. Want to give Contiguous a "Y" where Contig_ID is not NULL.

"Calculate Field for Contiguous"
Field Name: Contiguous

Expression: Answer(!Contig_ID!)

Code Block: 
def Answer(CI):
  if CI ==None:
    return "N"
    return "Y"

I've read plenty of posts explaining that NULL == None in python, but this code only returns Y for all numeric values in Contig_ID. Where Contig_ID == Null, no values are returned to Contiguous.

Any help would be grand!


Tags (2)
13 Replies
Occasional Contributor III
After some extensive testing, I can confirm this behaviour. If any of the inputs are <NULL>, it won't calculate for that row... Even when you get the row to calculate on itself.

That is weird though, as I'm sure I have done this before in Arc!

An annoying workaround would be to make a selection by attributes where Contig_ID IS NULL, set that to a marker value (i.e. 99999) and use that in your second calculation:
Expression: Answer(!Contig_ID!)

Code Block: 
def Answer(CI):
  if CI == 99999: # or '99999' if string...
    return "N"
    return "Y"

Let's hope the ESRI guys have an answer!
Occasional Contributor
Thank you Stacy,
Indeed, I am hopeful as well. Unfortunately, the null values are an indicator within the dataset that the client desires to maintain. Assigning another numeric value (0 or 999999) could potentially create miscalculations down the road.
Not to mention the messiness of creating a layer, selecting, and calculating a field based on data that are already present.

We are going to wait on this to hear if there is a solution.

Thanks all.

Occasional Contributor
75 people have viewed this thread. Has anyone else had any success in ArcGIS10 calculating a field in python based on a null value??

At the least, maybe this fix will make into SP3 due out next month?!
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Frequent Contributor
Only successful test I found was using a searchcursor, not field calculator. Null values must be read as something else in field calculator, because they come back as value True.
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New Contributor III
Hello Richard,

I've definitely been able to reproduce your issue and don't know what's going on.
However, the task you have in mind could be done trivially using Select by Attribute, followed by a standard Field Calculator expression.

In a Python script or in the Python window, it would be as simple as:
arcpy.MakeFeatureLayer_management('FEATURE_CLASS_NAME', 'temp', expr)
arcpy.CalculateField_management('temp', 'CALCULATED_FIELD', VALUE)

I don't have the exact figures, but this approach was significantly faster than comparing values one at a time through an unfiltered Update Cursor (and, by extension the way I understand it, any call to an CalculateField Python script.)

Doesn't change the fact that you're definitely pointing out some very weird behaviour that should be fixed!
Problem NIM070782 listed for Service Pack 3 might be this, but I'm not quite sure.
If you don't get a response from ESRI or someone knowledgeable in the next while, I'd recommend posting it on ArcGIS Ideas.

As an aside, I believe that the proper format should always be:
foo is None
and not
foo == None
because of:

  1. back-end optimization (which is also sadly why while 1 is preferred over while True); and

  2. the ability to overload an operator in classes and change the default behaviour of an == comparison.

It's no biggie, but worth pointing out. 🙂

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New Contributor II
Had this issue too... here's how got around it...

Expression: TestForNullOrEmpty(str(!FieldA!))

Code Block: 
def TestForNullOrEmpty(fieldVal):
  if len(fieldVal.strip()) == 0:
    return "FieldValueWasNullOrEmpty"
    return "FieldHadSomethingInItAlready"

The trick is to put   str(!FieldA!)    in as your parameter in the Expression, this will convert a Null or None value to an empty string.   Then by testing the length of the fieldVal that you passed and adding on the .strip()  you can then filter out all 3 of this scenarios with this code...

1. Field Value is Null (or None)
2. Field Value is empty string
3. Field Value is empty string but has some spaces in it (shouldn't happen but it does...)

Note however, that this code will not be capable of picking out "Only Null Values", because this code will treat Null's and Empty String as the same thing...

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New Contributor

All this solutions will work if you are on a edit session. You must Start editing the feature for the code to replace the value. 

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New Contributor


if not CI:

I was testing for None for Null field -- i.e.   if CI == 'None' for field returned with UpdateCursor

Even though a print statement returned 'None', couldn't be tested for as above.

if not ___

seems to work fine.


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MVP Esteemed Contributor

When cursors encounter NULL they return None and not 'None', which is a very important distinction.  The former is the Python None object while the latter is simply a string with the word None.  The expression CI == 'None' will never evaluate true for cursors returning fields with NULL. The proper way to check for a NULL field with cursors is CI is None